A difference or "differential" amplifier refers to an amplifier that produces an output that is proportional to the difference between two inputs (such as two input voltages or two input currents), and produces no output when the two input signals are equal. As is well known to those of skill in the art, differential amplifiers may be implemented using either bipolar junction transistors ("BJT"), field effect transistors ("FET") or other active device technologies. A typical BJT-based differential voltage amplifier comprises a pair of BJT transistors that are coupled at their emitters and biased by a current source that is connected to the coupled emitters. An input signal is applied to the base of each transistor, and the amplifier's output is the voltage measured between the collectors of the two transistors. FET-based differential voltage amplifiers are typically configured similarly, with the current source coupled to the sources of the transistors, and the input signals applied to the gates of the transistors. Those of skill in the art will appreciate that there are a wide variety of different types of differential amplifiers, including voltage, current, transconductance and transresistance amplifiers, and that each type of differential amplifier may be implemented in a wide variety of configurations.
Differential amplifiers are used in a wide variety of electronics applications, such as in stereos, radio receivers, telephones and the like. In many of these applications, it is desirable that the response of the amplifier be as linear as possible. An amplifier provides a "linear response" if the output of the amplifier is an exact replica of the input, except that it has a different magnitude (i.e., there has been no change in the input signal except for in its magnitude). One reason that a highly linear response may be required is because when two or more signals are input to a non-linear device, intermodulation (IM) power is created. These intermodulation effects may appear as interference to the desired signal. As is well known to those of skill in the art, intermodulation products appear at the frequencies corresponding to the different combinations of sums and differences of the frequencies of the signals input to the non-linear device. Thus, for instance, where two signals are input to the non-linear device, the center frequencies of the resulting IM product may be computed as: EQU Frequencies IM.sub.m,n =ABS[m(f.sub.1).+-.n(f.sub.2)] (1)
where m and n are integers, and f.sub.1 and f.sub.2 are the center frequencies of the respective input signals. The "order" of the intermodulation product centered at a particular frequency is the sum of m and n. Thus, in the case where two input signals, centered at 600 MHz signal and 1800 MHz respectively, are input into a nonlinear device, the second order intermodulation product will appear at 1200 MHz and 2400 MHz, and the third order intermodulation product will appear at 600 MHz, 3000 MHz and 4200 MHz, and so forth.
In communication systems that employ some type of frequency division multiple access ("FDMA") protocol, the user terminals may be configured so as to be capable of transmitting and receiving signals at a variety of different frequencies. Consequently, the amplifiers used in these systems may be designed to amplify signals across the entire bandwidth of the FMDA system, even though the desired signal may be of much narrower bandwidth. As a result, signals transmitted to other users in other channels of the FDMA communications system may be received by a user terminal, and amplified by various amplifiers included in the receiver. In these and a variety of other applications in which multiple signals are simultaneously input into an amplifier, it will typically be desirable to use an amplifier that provides as linear a response as possible, as any non-linearity will give rise to intermodulation distortion with its interference effects.
In certain radio receiver topologies, such as in homodyne receivers, the receiver operates on signals that are at or near baseband. In these systems, even-order non-linearity (i.e., intermodulation which has an "order", as defined above, which is an even number) may be especially troubling, because it typically creates interference at or near baseband. Balanced, or differential, circuits are typically used in such applications, as they may have reduced even-order non-linearity as compared to an equivalent single-ended circuit, as the even-order non-linearities theoretically are cancelled out. While it typically is difficult to achieve a perfect balance, it is often possible to significantly reduce the level of the evenorder non-linearities by using differential circuits.
In practice, the degree of balance of a differential circuit is largely determined by how well the components comprising the circuit are "matched." Thus, for example, to obtain a high degree of balance in a differential amplifier one should construct the amplifier using two transistors that have substantially identical electrical properties and performance. However, due to limitations in semiconductor fabrication techniques, it becomes increasingly difficult to fabricate transistors with identical electrical properties as the physical size of the transistors decreases. Consequently, mismatches in the size of the various parts of the transistors (i.e., base, connector and emitter) of 10% or more are common in many types of differential amplifiers. These mismatches may result in a significant increase in even-order non-linearity, which usually degrades the performance of the system in which the differential amplifier is used. In bipolar differential amplifier applications, mismatches in the size of the emitter area may be the most critical in terms of the generation of even-order intermodulation products, as the area of the emitter determines the current draw of the amplifier.
Presently, two different methods are typically employed to reduce even-order non-linearity in differential amplifiers. Pursuant to the first of these methods, the component mismatches, and hence the even-order non-linearity, is reduced by using relatively large transistors. This method of providing better component matching relies on the fact that the mismatches in the size of the emitters of the transistors forming the differential amplifier are primarily caused by limitations in etching, mask and other semiconductor manufacturing tolerances. As these tolerances are typically independent of the physical size of the device fabricated, by using larger devices the degree of the mismatch, as measured as a percentage, is reduced. Accordingly, by using larger devices, it is possible to reduce the relative size of the mismatch, which, in turn, provides a corresponding reduction in the even-order non-linearity of the device. Unfortunately, however, any increase in the size of the transistor also typically results in a corresponding increase in the parasitic capacitance of the device. These parasitic capacitances can both degrade the radio frequency performance of the device in which the differential amplifier is used and may also increase power requirements. Consequently, the use of larger devices may not be a viable option in applications with relatively large bandwidth requirements or in power limited systems.
The second method for reducing even-order non-linearity in a differential amplifier involves the use of resistive or inductive degeneration. Pursuant to this method, impedance is added between the emitters (or sources) of the differential amplifier, which acts to improve the overall linearity of the device. However, this reduces the gain of the amplifier, and hence to keep the gain at a certain level, it is necessary to increase the bias current, which results in increased power consumption and heat dissipation. Additionally, if resistive degeneration is used, the added resistance makes the device more noisy, which degrades the radio frequency performance of the device, and the use of inductive degeneration increases power dissipation even further. Moreover, with either type of degeneration, there are limits with respect to the degree to which the non-linearity can be reduced. Consequently, degeneration may also not be an acceptable method of reducing even-order non-linearities in certain applications.